1. Field of the Invention
The present invention relates to a method for determining pulmonary stress as well as to a breathing apparatus operating according to the method
2. Description of the Prior Art
Mechanical ventilation is used as a life saving treatment in many circumstances, but it can also aggravate pre-existing disease and even induce lung injury if the dynamics and physiology of mechanical breath delivery are not considered. The lung has an inherent tendency to collapse. During normal breathing this tendency is counteracted by the chest wall and a natural substance called surfactant.
In disease the collapsing-tendency becomes more pronounced, giving rise to areas (alveolar units) collapsing early during exhalation/expiration and opening late during inhalation/inspiration. This cyclic opening and closing of airways may initiate lung injury manifested as gross air leaks, diffuse alveolar damage, pulmonary oedema and pulmonary inflammation, all of which have been termed Ventilator Induced Lung Injury (VILI). The cyclical opening and closing of alveolar units can be counteracted by the administration of a correctly set Positive End Expiratory Pressure (PEEP).
A second postulated mechanism for VILI is the delivery of large tidal volumes (which can cause volutrauma) or high end inspiratory airway pressure (which can cause barotrauma). Both may over-stretch lung tissues, leading to fluid accumulation, inflammation and increased stiffness of the lung. Baro-/volutrauma can be avoided by setting a proper tidal volume or peak pressure.
If the ventilator settings are not optimized, the period before VILI is manifest can be considered as a period of increased stress. Hence, a determination of the degree of lung stress that may follow from a specific ventilator setting can be considered as a pulmonary stress index (PSI).
In European Application 1 108 391, a method and apparatus addressing these problems is disclosed. The method described in this published application is based on P-t measurements made during constant flow inspiration.